I think we just grew up. When I joined Facebook as a teenager in 2008, we essentially had no filter. Every thought, every photo, every relationship update - all of it was immediately shared to Facebook because at the time it was fun and novel and innocent.

For better or worse, millennials have become much more discretionary in what they post online than they were 15 years ago. I imagine Facebook had organic content from your real friends to show you then it probably would, the well's just run dry.